Networks that primarily utilize data link layer devices are often referred to as layer two (L2) networks. A data link layer device is a device that operates within the second layer of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer. One example of a common L2 networks is an Ethernet network in which end point devices (e.g., servers, printers, computers) are connected by one or more Ethernet switches or other L2 network devices. The Ethernet switches forward Ethernet frames, also referred to as L2 communications or L2 packets to devices within the network. As the Ethernet switches forward the Ethernet frames the Ethernet switches learn L2 state information for the L2 network, including media access control (MAC) addressing information for the devices within the network and the physical ports through which the devices are reachable. The Ethernet switches typically store the MAC addressing information in MAC tables associated with each of their physical interfaces. When forwarding an individual Ethernet frame, an ingress port of an Ethernet switch typically multicasts the Ethernet frame to all of the other physical ports of the switch unless the Ethernet switch has learned the specific physical port through which the destination MAC address devices is reachable. In this case, the Ethernet switch forwards a single copy of the Ethernet frame out the associated physical port.
Some layer three (L3) networks that route communications at the third layer of the Open Systems Interconnection (OSI) reference model, i.e., the network layer, employ L3 network devices that also perform L2 functionality to bridge and switch L2 communications to other L3/L2 and L2 network devices within the networks. In a typical configuration, provide edge (PE) routers coupled to customer network sites bridge L2 communications between the customer network sites on customer-facing interfaces and simultaneously route L3 communications over the L3 network core on core-facing interfaces. Each of the PE routers thus operates as a L2 switch having L2 customer- and L3 core-facing interfaces to connect the multiple LAN segments of what is, in some cases, an overall customer network defined by the individual customer network sites.
In some cases, a customer network site may be given redundant connectivity to a network through multiple PE routers. This form of redundancy is referred to as “multi-homing” and can be implemented in active-standby or active-active configuration. In active-standby multi-homing, one of the multiple PE routers coupled to the customer network is traditionally chosen as the active PE router, or designated forwarder, to send traffic to and from the customer network. The other one or more PE routers are designated as backup forwarders which can be used to send traffic to and from the customer network in the event of a network failure that would preclude the current designated forwarder from sending the traffic. In active-active multi-homing, each of the multiple PE routers by which the customer network site has connectivity to a network actively bridges traffic to and from the customer network.
In both active-standby and active-active redundancy configurations, the multiple PE routers that provide redundant connectivity traditionally execute a virtual router protocol, such as Virtual Router Redundancy Protocol (VRRP), to present a uniform L2 interface in the form of a virtual gateway L2 address to one or more customer networks attached to the PE routers. Hosts of the customer networks may address packets for routing external to the L2 domain to the virtual gateway L2 address to identify such packets to the PE routers, which then route the packets according to a routing instance toward the L3 core network.